Interpretive Summary: An increasing number of food-borne illness outbreaks associated with the consumption of packaged fresh-cut products calls for attention for maintaining food safety. Storage temperature is a critical factor for maintaining produce quality, and shelf life; yet its impact on produce safety is unclear. This study examined the effect of temperature on the growth of disease-causing microorganisms on commercially packaged young spinach leaves. We demonstrated that there is a significant increase in their populations of disease-causing microorganisms when the spinach leaves were held at elevated temperatures while there is no increase in its populations when the spinach leaves are held at 41 °F or below. This suggests that proper refrigeration is important to limit the growth of disease-causing microorganism and to maintain food safety. The outcome of this research provides avenues of intervention and mitigation of food-borne illnesses on fresh and fresh-cut produce. This information will be useful to the Fresh-cut processing industry to help insure that safe produce is provided to the consumer.

Technical Abstract:
This study evaluated the effect of storage temperature and time on the survival and growth of Escherichia coli O157:H7, the growth of indigenous microorganisms, and the changes in product quality of packaged baby spinach. Commercial packages of spinach within 2 days of processing were cut at one end, sprayed with fine mists of E. coli O157:H7 inoculum, re-sealed, and stored at 1, 5, 8 and 12 °C for 12 days until their labeled “Best If Used By” dates. Microbial enumeration and product quality evaluation were conducted on days 0, 3, 6, 9 and 12 post-inoculation. Spinach held at 12 °C supported significant (P < 0.001) E. coli O157:H7 growth, with a 1.0 log cfu/g increase within 3 days post-inoculation, followed by additional growth during continued storage. E. coli O157:H7 grew slowly on spinach held at 8 °C, with a significant level (P < 0.01) of growth reached after 6 days of storage. However, E. coli O157:H7 populations declined significantly on products held at 1 and 5 °C (P < 0.01 and P < 0.001, respectively) within 3 days of storage. Aerobic mesophilic bacteria, psychrotrophic bacteria, and yeast and mold populations increased significantly at all storage temperatures tested, with more growth on products held at higher temperatures. Products stored at all temperatures maintained high quality scores within the first 6 days in storage, although there was a sharp decline of quality scores on spinach stored at 12 °C noted on day 9 post-inoculation. These results suggest that E. coli O157:H7 can grow significantly on spinach held at 8 °C or above, before significant product quality deterioration occurs. Refrigeration at 5 °C or below can limit the growth of E.coli O157:H7 and mitigate risks associated with potential pathogen contamination and proliferation.